Effects of the land disposal of water treatment sludge on soil physical quality.

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Abstract

An essential step in producing "drinking" water is to precipitate the suspended and dissolved
colloids through the addition of flocculents such as lime, ferric chloride, aluminium sulphate
and/or poly-electrolytes. The by-product of this process is termed water treatment sludge
(WTS) and contains mainly silt, clay and some organic matter. Previously this material was
disposed of in landfill but more recently, alternative methods for its disposal are being
evaluated. A potential disposal option is land treatment. In this system of waste disposal the
inherent properties of the soil are used to assimilate the waste. Although the effect of the land
disposal of WTS on soil chemical quality is gaining increasing research attention, few studies
have investigated the effects on soil physical quality.
This study was originally commissioned by a local water utility to evaluate the effects of the land
disposal of sludge produced at their works, on soil quality. At this plant organic polymers are
used to both flocculate the material and to thicken the sludge in the water recovery process.
Fresh sludge has a consistence approaching that of slurry but dries to angular shaped aggregates
of extremely high strength. Nevertheless, sludge aggregates comprise a network of micro-pores
and channels and are therefore porous. Because of these properties, the potential use of WTS
as a soil conditioner was considered.. Since lime, gypsum and polyacrylamide are wellrecognised
soil conditioners, these were included as reference treatments in the study.
Two field trials (Brookdale and Ukulinga) and laboratory experiments were designed to
investigate the influence of WTS on soil in terms of water retention, hydraulic conductivity,
evaporation, aeration, aggregation and strength. Seven rates of WTS are represented at the .
Brookdale trial but research efforts were concentrated on the 0, 80, 320 and 1280 Mg ha'
treatments. WTS was also applied as a mulch (without incorporation into the soil) at the 320,
640 and 1280 Mg ha" level. Gypsum was applied at rates of 5 and 10 Mg ha", lime at 2 and
10 Mg ha' and anionic polyacrylamide at 15 and 30 kg ha'. At the Ukulinga trial, WTS was
mixed with the upper 0.2 m of the soil at rates of 0, 80, 320 and 1280 Mgha'. Only the high
rates of gypsum, lime and anionic polyacrylamide being tested at the Brookdale trial are
represented at the Ukulinga trial. All treatments in this study were maintained fallow. The laboratory study features an additional two soils to those from the field experiments, chosen
to produce a range in clay contents.
WTS influenced several soil physical properties. Soil bulk density decreased following the
addition of sludge to soil. This caused an increase in porosity (particularly macro-porosity) and
therefore water retained at saturation, but only of statistical significance at the 1280 Mg ha"
level. Equally an increase in water retention at the wilting point (-1500 kPa matric potential)
also occurred, owing to the high microporosity of sludge aggregates. Despite these effects very
little change in both the plant available and readily available water content occurred. Neither,
gypsum nor lime caused any significant change in water retention. Aslight improvement was
noted on the polyacrylamide treatment at the Brookdale site but this effect did not persist for
very long after the trial was established.
Although in situ field measurements were influenced strongly by natural spatial variability,
WTScaused a marked increase in the saturated hydraulic conductivity (Ks). The reasons for this
relate to the higher porosity and the inherently stable nature of the sludge aggregates, which
imparts a more open structure to the soil and reduces the extent of pore blockage. This finding
was corroborated in a laboratory study in which strong positive correlations between sludge
content and Ks was found. The water retention curve and saturated hydraulic conductivity was
used to predict the unsaturated hydraulic conductivity function (Kw)using the RETe computer
model of van Genuchten et al., 1991. The results showed a decrease in Kw on the sludgeamended
treatments the extent of which increased with sludge content. This finding was tested
in an evaporation study conducted under controlled environmental conditions. More water was
conserved on the sludge-amended treatments than the control, because of its lower Kw. The
application of the sludge as a mulch was more effective in conserving water than incorporating
the sludge with soil.
The air-filled porosity at field capacity (-10 kPa matric potential) of the sludge-amended soil
remained within a favourable aeration range of 10-15%, which suggests that aeration should
not be a limiting factor for plant growth. Air-permeability nevertheless improved substantially.
Attempts at using the size distribution of dry soil aggregates to evaluate the influence of the
sludge on aggregation proved unsuccessful. Saturated soil paste extracts for selected soil depths beneath the mulch layers at the Brookdale trial, nevertheless, showed significant increases in
Ca2+ and Mt+ concentrations, which is encouraging from a soil stability perspective. Due to
the inherently strongly aggregated nature of this soil, no meaningful change in aggregate
stability, however, was measured. Significant improvements in soil stability were, nevertheless,
found when fresh sludge was mixed with soil. If the sludge is not allowed to dry fully
beforehand the polymer that it contains remains active and available for bonding of the soil
particles together. Upon drying, these polymers become irreversibly attached to the soil
substrate and win not become reactivated even upon re-wetting of the soil. This also explains
why sludge aggregates found below only a few centimetres of the soil surface maintained their
strongly aggregated nature. This suggests that although WTS consists of mainly silt and clay,
the risk of this constituent fraction becoming released and clogging water conductive soil pores
are, at present, low. Despite the high strength of the sludge aggregates the penetrometer soil
. strength (PSS)within the tilled layer was non-significantly different from the control treatment.
Below the tilled layer, however, the PSS on the sludge-amended treatments were lower owing
mainly to wetter soil conditions.
The research completed to date suggests that land treatment as an environmentally acceptable
disposal option for water treatment sludge shows promise since soil conditions tend to be
improved.

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